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**s/v Jedi** · 26-10-2009, 15:40

Mainsail will surely bond that fiberglass "block" to the hull with epoxy and it won't break free from the hull after that.

Whenever you put two things together and the mechanical fasteners used can cope with the forces, you are better off with a polysulfate like 101 or lifecaulk. The sealant has no role in bonding in that case and doesn't need to be an adhesive.

About threading failure: from memory I think for fiberglass the length of the thread must be 1.5 x the diameter of the fastener. In cast epoxy it is more, like 2-3 x the diameter. From looking at Mainsail's photo's I think he's at least at 2 x the diameter so no worry there.

cheers,
Nick.

Cheechako · 26-10-2009, 15:56

It's an interesting way to do it, I'm just thinking it thru.... just not sure there is any advantage really....Yea, usually the catastophic failure of a seacock assembly is the corrosion of the threaded portion of the thru hull fitting right? (thinnest portion of the whole assembly and the thread root being a perfect spot for corrosion to start also). Thus the historical "proper" way is to bolt thru the hull.... which puts no strain on the fitting when trying to open a frozen valve. If you are going to epoxy to the hull and rely on that bond, then I guess there would be nothing wrong with 5200 either... both will be very hard to remove! Hmmmm.... buried bolts....bolts are now permanently embedded if you break one, cross thread or they corrode.

Maine Sail · 26-10-2009, 17:09

Quote:

Originally Posted by Cheechako

Mainsail: Very nice looking blocks.... but you are bolting your seacock just to the block?

Yes I am in that application. I wrote that piece because there are a bunch of folks who have not been around boats as long as others and who are afraid or timid of the traditional and proven "extra holes" in the hull. For them this approach gives piece of mind.

This is a very robust design but I still through bolt personally though I may do more of these as it is less labor intensive and still very, very strong. That particular seacock is very, very well protected on my boat so I chose to tap the backing block. The mast base on my boat is also tapped into solid fiberglass. It can see tremendous loads when compared to a seacock. It's now 30 years old and has not budged one iota of a mm...

Quote:

Originally Posted by Cheechako

I guess that's a little better than no bolts... I kind of like it as a compromise.... but realistically, you could end up with a seacock/block broke free from the hull if the threads fail..... just like with no bolts right?

No actually it's a LOT stronger than no bolts. I've rarely heard of failed threads on a prpperly flanged seacock. If you use like componenets, eg: all 85-5-5-5 bronze, there should be little to no risk of disimilar metals corrosion. The ball in the valve will usually go first well before the thru-hull threads. When thru-hulls fail it is usually because it was not a proper flanged seacock but rather a valve on a stem..

Even without bolts a flanged seacock spreads the load better and absorbs much of what would be directed at the thru-hull had there been no flange at all. These backing plates should easily meet the 500 pound static load requirement of ABYC. I actually had a friend who's a mechanical engineer run the numbers. It is a LOT stronger that one would think.

Quote:

Originally Posted by Cheechako

I guess you get a little more "bonding area" if you are using 5200..... but with 101 it's probably not as strong as 5200 and no bolts.....

Sure, but those backing blocks are permenently glassed to the hull not glued with a sealant. Because I use flanged adapters (best invention since the wheel) and can replace a valve by simply un-threading it, the flanged adapter is a permanent item not likely to ever need replacing.

Remember this is just another option! It is also an option that is far better and stronger than the hundreds of thousands of boats out there with a valve slapped on a thru-hull with no flange, no through bolts and mis matched threads to to it all off.

P.S. Morris Yachts does not even use thru-hulls. On their newer hulls they simply through bolt a Marelon seacock directly over a hole in the hull. The backing block and area are solid glass but they insert no thru-hull into the seacock so the hull exterior is super smooth for water flow..

Cheechako · 27-10-2009, 08:48

I kind of like it, never liked those 3 holes in the hull in close proximity to a larger hole! Ref: "threaded thru hull breaking..." I really dont know how else a seacock setup would fail catastrophically....? If the stem of the thru fitting doesnt break, than all is going to be well and the bolts aren't needed right?. (measure the I.D. of a thru hull sometime and then measure the depth of thread... I bet some arent more than about 1/32"-1/16" thick...or a little more) I know of one case where the thru hulls were completely turned to light green power in about 3 months time! Here in Seattle, a diver went down to clean the bottom on a boat that had returned from a world cruise with no issues. (After returning, the bottom had been done and the prop shaft replaced.) After 3 months in a Hot marina situation .... the diver came up and told the owner to have the boat hauled immediately. I saw some of the sea cocks and prop shaft, the sea cocks had the broken off stem of the thru hull still in them. You could crumble the bronze with your fingers! The prop shaft looked like a lace tablecloth!

cburger · 27-10-2009, 10:41

Boy has this thread gotten a little beserko of what? Whilst I appreciate obsesive compulsive behavior as much as anyone, we are only talking about fabricating a backer donut for a through hull. I believe the original question had to do with re-habing an existing wood backer with an epoxy based product, (Git Rot). The simple answer is the rotted wood has to be replaced, the through hull and seacock removed, inspected and serviced and the whole enchilada re-installed with a proper bedding compound, (There are many).

Wood has been the choice for years for this application, I am not saying there are not other materials that are applicable, (Titanium), for me it’s just a pragmatic time to reward/ cost ratio. For the average cruising boat I can take a piece of scrap marine ply 1/2" laying around my shop and with a saber saw cut a new donut in about 60 seconds, a little West System for coating against water intrusion and I am done. Life expectancy easily 30years+, considering I won't be alive then what’s the question. Time is to precious, I would rather spend my time watching the Yankees win the World Series